Suspension assemblies are spring structures in disk drives that position a read/write head assembly nanometers away from the rapidly spinning surface of a rotatable disk or other data storage device. The suspension assembly presses the head toward the disk surface with a precise force applied in a precisely determined location. The head assembly "flies" over the irregular (at this scale) surface of the disk at a height established by the equilibrium of the suspension assembly downward force and the increasing lift force of the air stream generated by the spinning disk as the head nears the disk.
A head suspension assembly (HSA) includes the suspension assembly, the head assembly, and an interconnect assembly. The interconnect assembly is a collection of transmission elements designed to transmit data to and from the head assembly. HSAs are used in magnetic hard disk drives, the most common today, or other type of drives such as optical disk drives.
The suspension assembly includes two spring structures, a load beam and a gimbal, each a carefully balanced combination of rigid regions and flexible spring regions. The load beam is a resilient spring plate designed to provide lateral stiffness and calibrated to apply the necessary load on the head assembly. The gimbal is a spring positioned at the distal end of the suspension assembly and of the load beam. The gimbal holds the head assembly at an appropriate orientation (flying attitude) and at a constant distance over the contours of the disk, even if the load beam flexes and twists. The head assembly is attached to the gimbal and includes a "head," a highly sensitive read/write transducer, attached to an air bearing slider. The head assembly also includes electrical terminals configured for interconnection to the interconnect assembly for receiving and relaying data (read and write signals).
A magnetic write transducer transforms electrical write signals into small magnetic fields. The magnetic field magnetize patterns on a magnetic disk. The order of the magnetic fields and their subsequent orientation defines a bit code representing the stored data. A magnetic read transducer "reads" these magnetic fields as it flies over them and converts them back into electrical signals.
The suspension assembly can be attached at its proximal end to a rigid arm or directly to a linear or rotary motion actuator. The actuator rapidly moves (and then abruptly stops) the HSA over any position on a radius of the disk. The radial HSA movement and the rotation of the disk allow the head to quickly reach every location above the disc. The rapid stop and go movement causes very high stresses on the HSA.
The closer the head assembly can fly to the surface of a magnetic disk, the more densely information can be stored (the strength of a magnetic field varies proportionally to the square of the flying distance, thus the smaller the head's flying clearance the smaller the magnetic "spot" of information). Manufacturers of disk drives strive to reach flying clearances close to 100 nanometers=0.1 micrometers (a human hair is about 100 micrometers thick). However, the head assembly must not touch the disk ("crash"), since the impact with the spinning disk (rotating at about 3600 rpm or faster) can destroy both the head, the surface of the disk, and the stored data.
Amplifying and control electronic circuits process, send, and receive the data signals to and from the head assembly. Signal transmission requires conductors between the dynamic "flying" head and the circuitry. Traditional head assemblies complete a read/write circuit loop with two conductors, usually copper wires encapsulated in a plastic sheeting. Newer magneto-resistance head assemblies require four or more independent conductors. The interconnect assembly includes the conductors and accompanying insulators and connectors.
Designers and manufacturers of HSAs face competing and limiting design considerations. During operation, the suspension assembly should be free of unpredictable loads and biases which alter the exact positioning of the head assembly. The suspension assembly should respond instantaneously to variations in the surface topology of a disk. Alterations to the flying height of the head can significantly affect data density and accuracy and even destroy the system in a crash.
Rigidity and stiffness increase in relation to the third power of cross-sectional thickness. To respond to air stream changes and to hold the flying head at the appropriate orientation, suspension assemblies are very thin and flexible, specially around the sensitive spring and gimbal areas. Interconnect assembly conductors have a large effect on suspension assembly performance. Conductor stiffness alone greatly affects the rigidity of the spring regions and flight performance. A standard conductor placed atop of a thin suspension can more than double a spring region's stiffness and detract from the ability of the spring region to adjust to variations in the surface of the disk, vibrations, and movement. The effect of the conductors on a gimbal region, the thinnest and most delicate spring in the suspension assembly, is even more pronounced. Conductors placed over spring regions must not plastically deform (become permanently bent) when the spring regions flex, since such deformation hinders the return of the spring to its normal position and applies a load on the suspension assembly.
The ideal HSA comprises components low in mass. Excessive inertial momentum caused by excessive mass can cause overshoot errors. Overshoot errors occur when momentum carries the whole HSA past the intended stopping point during positioning movement. Low-in-mass HSAs are also easier to move, resulting in power savings in multiple platter disk drives.
The manufacture of HSAs, like that of any commercial product, must be efficient. Reduction of manufacturing steps is desired. Damaged or misaligned components introduce biases and loads and drastically diminish the manufacturing useful output yield. Complex shaping and mounting processes are costly and decrease the reliability of the whole HSA manufacturing process.
To avoid defects and unpredictable loads and biases, exacting tolerances are necessary. During the HSA manufacturing and assembling process, the buildup of deviations from tolerance limits causes planar deviations that can affect the flying attitude of the head assembly. The parameters of static roll and static pitch torque in the final HSA result from these inherent manufacturing and assembly tolerance buildups.
Mounting and placement of current interconnect assemblies is usually done by hand. Hand mounting is imprecise and costly. Precise conductor placement is specially critical in the delicate gimbal region. As the industry transitions to smaller slider/transducer sizes to increase data storage density, limitations of the current interconnecting devices increase the potential for read/write errors and impose ceilings on data storage density.
Using current interconnect technology, workers bond two to five lengths of wire to the head assembly, using fixturing to manage the wires while adhesively bonding the head assembly to a stainless steel suspension. Next, the lengths of wire are shaped by hand, using tweezers and tooling assistance to form a service loop between the head assembly and the suspension assembly and to position the wire along a predetermined wire path on the suspension assembly. The wires are tacked to the suspension using adhesive or wire capture features formed into the suspension. Special care is taken to avoid pulling the service loop too tight or left too loose. A tight service loop places an unwanted torque on the slider causing flying attitude errors. Loose service loops allow the wire to sag down and scrape on the spinning disk. Both conditions are catastrophic to drive performance. Through-out the process of handling the slider and wires there is a risk of damaging the wires or the delicate load beam and gimbal. Load beams or gimbals accidentally bent during the manufacturing operations are scrapped. Often the head assembly also cannot be recovered, adding additional losses to the scrap pile.
Another type of suspension assembly interconnect utilizes plastic compounds acting as integral spring elements in the suspension assembly. Use of plastic materials as spring elements in load beam and gimbal construction presents performance problems since plastic materials do not possess optimal mechanical spring qualities. As the flying height and head size continually decrease in the progression towards greater disk storage density, the accuracy and control needed to align the transducer to the correct data track upon the disk surface becomes more critical.
During operating conditions the drive temperature operating ranges can span 80 degrees Celsius. Plastics expand and contract more than metals during temperature changes. The use of thermally expansive plastics as principal spring structural elements of the load beam or gimbal region affects the dimensional stability of the suspension assembly. The expansion and contraction of integral plastic spring elements introduces loads and stresses on the metal components. Additionally, because of their mechanical characteristics, integral plastic spring elements traditionally only have been used in suspension assemblies that resist the pull of negative pressure sliders, sliders that create a vacuum that pulls them toward the disk.